The invention relates to a shackle element, to a method for making a shackle element, a hook with a shackle element and a rode connector with a shackle element.
Hooks and rod connectors are employed in orthopedic medicine for patients who, for example because of spondylosis, temporarily or permanently require an internal fixation device. Within the framework of the present specification, the term hook is intended to include all such hooks, for example pedicle hooks, lamina hooks, lumbar hooks and transversal hooks, wherein the latter are employed as safety hooks, for example for pedicle hooks. By means of internal fixation device it is attempted for the patient to be free of symptoms, even though it must be accepted that his mobility is restricted. In general, one fixation rod per side of the body and essentially extending in the direction of the spine is provided, wherein each fixation rod is equipped with at least two hooks. The two fixation rods are coupled with each other by approximately horizontally extending transverse rods, which are also identified as transverse connectors, wherein respectively one rod connector is used for the mutual connection of the fixation rod and transverse rod.
The hook has an actual hook element, which from the back grips a laterally extending portion of the spine from underneath or above, and a shackle element produced integrally with the hook element, which is used for receiving and fastening the fixation rod.
The rod connectors have a clamping device for fastening on the transverse rod, and a shackle element for clamping the fixation rod, wherein the clamping device for the transverse rod can be designed the same as or differently from the shackle element for the fixation rod.
The shackle element which is essentially designed the same for the hook and the rod connector, forms a groove extending in the direction of the fixation rod, having a groove bottom and two laterally connected lateral walls. When affixing the internal fixation device the process is such, for example, that the required hooks per body side are attached, then the fixation rod with the rod connectors is introduced laterally into the groove formed by the shackle element, thereafter the transverse rods are attached and fixed in place, and finally the hooks and rod connectors are definitely fixed in place on the fixation rod.
A perfect mutual connection between the hooks, or respectively the rod connectors, and the fixation rod is of decisive importance for the perfect functioning of such an internal fixation device. This connection cannot be permitted to become loose either by the effects of heat or by mechanical stresses, and the creation of abrasion particles must be prevented.
In connection with a known hook, the shackle element is designed in such a way that a screw is screwed into a lateral wall of the groove which is embodied to be flat, because of which the front of the screw bolt is pressed on the fixation rod, so that the fixation rod is squeezed, or respectively clamped, between the screw bolt and the opposite lateral wall. It is obvious that such fastening is unsatisfactory. On the one hand, the fixation rod can be displaced comparatively easily along the flat lateral wall transversely in respect to its longitudinal axis and, on the other hand, a very high pressure is exerted at the place of the practically only point-like contact between the screw bolt and the fixation rod, which favors the creation of abrasion particles.
In connection with another known hook, whose shackle element corresponds to that of the prior art mentioned at the outset, this shackle element forms a groove with a groove bottom which is semicircular in cross section and which continues on both sides into lateral walls which are parallel to each other. Here, too, fastening is provided by a screw acting on the surface of the fixation rod. The extension of the longitudinal axis of the screw does not lead radially through the respective cross section of the fixation rod, but in the direction of a secant of this cross section, namely in the area of the half of the cross section not in contact with the groove bottom. An improvement in comparison with the first described known hook is achieved, because a displacement of the fixation rod transversely to its longitudinal direction is prevented by means of the screw, which in respect to the fixation rod is eccentrically arranged. But, as already mentioned, in connection with this hook fastening of the fixation rod in the fastening element is also provided by means of a screw whose front is pressed directly on the fixation rod, which can lead to the creation of abrasion particles.
In summary, it can be stated that no shackle elements for hooks and rod connectors are known which are designed in such a way that a connection, which is perfect in every way, with the fixation rod is assured.
It is therefore the object of the invention to
improve the shackle element of the type mentioned at the outset in such a way that the mentioned disadvantages are avoided,
propose a method for making the shackle element;
propose an improved hook and
propose an improved rod connector.
These objects are attained in accordance with the invention by the features of the independent claims. Dependent claims define preferred embodiments of the shackle element, of the method for making the shackle element, and of the of the hook.
The novel shackle element is embodied in such a way that it forms a groove, whose groove bottom in cross section not only extends along an arc with an angle of 180xc2x0 but also along an additional narrow partial angle, so that the fixation rod is enclosed by the groove bottom over more than half its circumference. When making the shackle element, the two lateral walls adjacent the groove in general are first parallel, whereby the distance between them is less than the diameter of the fixation rod. Then the walls are spread apart from each other, so that there is a small angle between them and their minimum distance at the place where they meet the groove is slightly larger than the diameter of the rod. This allows for easy transverse mounting of the rod. Then the clamping of the fixation rod in the shackle element by means of the screw device takes place by squeezing the two lateral walls of the groove again towards each other. As a result a displacement of the fixation rod transversely in respect to its axis is prevented by the novel shape of the groove, and the force exerted during the clamping of the fixation rod acts along a considerable surface, so that large point-like stresses which might lead to fissures are avoided.
As already mentioned the shackle element may be constructed such that the distance of the lateral walls from each other is slightly less than the diameter of the groove, or respectively of the fixation rod, so that the lateral walls directly follow the opening of the groove bottom. However, it would also be possible to select the distance of the lateral walls from each other corresponding to the diameter of the groove bottom, or respectively of the fixation rod, wherein the lateral walls then would start at a lesser distance from the opening of the groove bottom.
The screw device is preferably designed in such a way that the one of the lateral walls of the groove has an interior screw thread, and the other of the lateral walls of the groove has a through bore. For clamping the two lateral walls together, a cap screw or a screw bolt with a nut is screwed through the through bore into the interior screw thread.
Two requirements must be considered when determining the screw-in depth, or respectively the number of the screwed-in screw turns: on the one hand, the shackle element should have the lowest possible volume in order to minimize muscle problems for the patients, on the other hand, a certain screw-in depth must be provided in order to achieve a sufficient rigidity of the screw connection. It has therefore been found to be advantageous to design the lateral wall with the interior screw thread thicker in the direction of the longitudinal thread axis than the lateral wall provided with the through bore.
In order to secure the screw connection also without the use of an additional securing element, the exterior surface of the lateral wall provided with the through bore, against which the screw cap of the cap screw or the nut rest in the assembled state, can have one, or possibly several small protrusions, which are compressed when the screw connection is tightened and therefore exert a resilient counter-pressure on the screw cap or the nut, so that the screw connection is secure against being loosened.
It was shown to be advantageous, in particular as an aid in assembly, to fix the distance of the longitudinal thread axis from the center of the circle, which can be drawn in the groove bottom, or respectively from the longitudinal axis of the fixation rod, in such a way, that it corresponds to the sum of the radii of the mentioned circle which can be drawn in, and of the screw bolt, the latter preferably being without threads in this area. Because of this the fixation rods acts as a guide for the screw while it is being screwed in, and in the assembled state the screw bolt rests against the fixation rod.
Besides a shackle element in accordance with the invention, the hook of the invention has an actual hook element, which is intended to extend underneath or over an essentially laterally extending portion of the spine of a patient.
In accordance with the use of the hook, the actual hook element is designed in such a way, that the hook is a pedicle, lamina, lumbar or transversal hook, and embodiments of all hooks which can be used on the right side as well as on the left side are produced in straight as well as in angled variants.
Besides a shackle element in accordance with the invention, the rod connector of the invention has a clamping device for a transverse rod.
The hook, or respectively the rod connector, is intended to be used, together with a fixation rod on which it is fastened by means of the screw device, as a portion of an internal orthopedic fixation device, wherein the lateral walls of the shackle element, whose distance from each other generally is slightly less than the diameter of the fixation rod, are spread apart by plastic deformation prior to inserting the fixation rod, preferably already when the hook is made
Then, in the course of tightening the screw device, the lateral walls are elastically, and possible also plastically deformed.
For the purpose of preventing tension elements, all components of the internal fixation device, i.e. the hooks, the screw device and the fixation rod, are preferably made of the identical material, wherein titanium-niobium alloys have been shown to be particularly advantageous.
Further properties and advantages of the invention will be extensively described by means of exemplary embodiments, making reference to the drawings.